The emergence of a complex pore‐canal system in the dermal skeleton ofTremataspis(Osteostraci)

Abstract

Thyestiids are a group of osteostracans (sister-group to jawed vertebrates) ranging in time from the early Silurian to Middle Devonian. Tremataspis is unique among thyestiids in having a continuous mesodentine and enameloid cover on its dermal elements, and an embedded pore-canal system divided into lower and upper parts by a perforated septum. The origin of this upper mesh canal system and its potential homology to similar canal systems of other osteostracans has remained a matter of debate. To investigate this, we use synchrotron radiation microtomography data of four species of Tremataspis and three other thyestiid genera. Procephalaspis oeselensis lacks an upper mesh canal system entirely, but Aestiaspis viitaensis has partially enclosed upper canals formed between slightly modified tubercles that generally only cover separate pore fields. Further modification of tubercles in Dartmuthia gemmifera forms a more extensive, semi-enclosed upper mesh canal system that overlies an extensive perforated septum, similar to that found in Tremataspis. Lower mesh canals in P. oeselensis are radially arranged and buried tubercles indicate a continuous growth and addition of dermal hard tissues. These features are lacking to varying degrees in the other investigated thyestiids, and Tremataspis probably had a determinate growth accompanied by a single mineralization phase of its dermal hard tissues.The previously proposed homology between the semi-enclosed upper canal system in Dartmuthia to the pore-canal system in Tremataspis is supported in this study, but the suggested homologies between these canals and other parts of the thyestiid vasculature to those in non-thyestiid osteostracans remain unclear. This study shows that three-dimensional modeling of high-resolution data can provide histological and structural details that can help clarify homology issues and elucidate the evolution of dermal hard tissues in osteostracans. In extension, this can give insights into how these tissues relate to those found among jawed vertebrates.

Exploring the three-dimensional vasculature of dermal hard tissues in thyestiid osteostracans using synchrotron radiation microtomography

“…The emergence of this pore-canal system was the focus of a recent study by Bremer et al (2021), who affirmed its close relation to the partially enclosed canal system formed by modified tubercles in the other thyestiid D. gemmifera . However, that study concluded that it was not possible to determine its homology to inter- and intra-areal canals outside of Thyestiida (Bremer et al, 2021). Furthermore, the study agreed with the notion presented by Gross (1961), that the suggested radial canals in D. gemmifera (Wängsjö, 1946) could not be viewed as homologues to radial canals proper (Stensiö, 1927, 1932) because of their much higher position in the middle layer.…”

“…This study is based on the same dataset that was in part presented by Bremer et al (2021). All specimens were imaged using propagation phase-contrast synchrotron X-ray micro-computed tomography (PPC-SRµCT) at the ID19 beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France).…”

“…O'Shea et al (2019) later studied the dermal skeleton of an articulated specimen of Tremataspis mammillata using synchrotron radiation X-ray tomographic microscopy. More recently, Bremer et al (2021) focused on the emergence of the complex pore-canal system found in Tremataspis by using PPC-SRµCT on dermal elements from seven different thyestiid species. The latter study also identified some potential primary homologies of the vasculature among the selected taxa.…”

“…32B and 35A). More recently, novel approaches have been implemented to investigate the 3D structure of the thyestiids Oeselaspis and Tremataspis (O'Shea et al, 2019; Qu et al, 2015), as well as a few other thyestiid taxa (Bremer et al, 2021). These studies generally agreed with previously proposed models, but also highlighted some discrepancies.…”

High-energy synchrotron-radiation-based X-ray micro-tomography enables non-destructive and micro-scale palaeohistological assessment of macro-scale fossil dinosaur bones

“…For example, through a series of virtual-palaeohistological studies, ossification patterns, growth histories and geometries of muscle attachments are assessed in the humeri of fossil fish and early tetrapods (Sanchez et al, 2012(Sanchez et al, , 2013(Sanchez et al, , 2014(Sanchez et al, , 2016Estefa et al, 2020). Some additional studies also focus on Devonian fossil fish to understand the palaeobiology of the early vertebrates through virtual palaeohistology (Giles et al, 2013;Qu et al, 2015;Vas ˇkaninova ´et al, 2020;Bremer et al, 2021). To date, virtual palaeohistology is extensively employed in studying various aspects of fossil hominids, including diet, dental development, life histoly and taxonomy (Smith et al, 2007;Smith et al, 2010;Tafforeau et al, 2012;Le Cabec et al, 2015;Smith et al, 2015;Smith et al, 2018;Beaudet et al, 2021) (see Table S1 of the supporting information for a list of selected studies applying virtual palaeohistology to fossil vertebrate skeletons).…”